The December 2012 bulletin, obtained by ABC News, warned that "high ranking members of the Aryan Brotherhood of Texas are involved in issuing orders to inflict 'mass casualties or death' to law enforcement officials involved in the recent case."

The bulletin said "the plan is designed to be carried out when law enforcement officers are at large gatherings." It warned that the Aryan Brotherhood of Texas was "proactively working toward developing personal information about officers involved in the recent arrest of Aryan Brotherhood of Texas members," and was carrying out surveillance of the officers.

Law enforcement sources have said the Aryan Brotherhood of Texas, a violent prison gang, is a top focus of an investigation into the deaths of the prosecutors, Mark Hasse and Mike McLelland.

The case against the gang is expected to go forward this summer or fall.

Thirty-four Aryan Brotherhood of Texas members were indicted in Houston in November for crimes including racketeering, drug distribution and kidnapping. Ten members potentially faced the death penalty as prosecutors accused them of ruthless violence, including ordering hits on rival gang members.

A review of the case showed that two slain Kaufman County prosecutors assisted in the investigation, along with more than a dozen agencies.

Police have not confirmed a link with the Aryan Brotherhood of Texas with the killings, and are poring through hundreds of cases on which Hasse worked, running down leads that point to Mexican drug cartels, local drug traffickers and other violent individuals.

On Wednesday, the Kaufman County Sheriff's Office announced the arrest of Nick Morale for making terroristic threats against county officials.

Morale was being held on $1 million bond but, according to police, there is "nothing to link Mr. Morale to the murders" of Hasse and McLelland.

Police said they were also investigating other threats made to county officials, but would not provide details.

Judicial officials in Kaufman County have been given special protections. Judge Bruce Wood said this week that he was given a bodyguard, and sheriff's deputies have been parked outside the home of Brandi Fernandez, the county's new acting district attorney, since her appointment on Monday.

An assistant U.S. attorney prosecuting the Aryan Brotherhood of Texas has withdrawn from the case, citing "security purposes," just days after McLelland was killed in his home.

Jay Hileman filed a notice to withdraw from the case with the court, and on Tuesday sent an email about his decision to the defense lawyers representing 34 indicted members of the Aryan Brotherhood of Texas.

"He sent an email to all counsel that he was withdrawing for security purposes," defense lawyer Richard Ely said. "I'm not going to speculate why, but I can understand why someone with a family might withdraw, yes."

Another source familiar with the message told ABC News it was "abundantly clear" Hileman was stepping down because of security concerns.